It is common for an electronic printer to use a scanning laser light source and a rotating polygon to generate a beam to impinge on the photoconductor. The laser beam strikes the photoconductor and, in the well known manner, discharges the drum, leaving a charge pattern on the surface of the photoconductor in the shape of the information to be printed. Then the drum is developed with toner which is transferred to an output document.
A problem to be considered is scanning speed. Rotating the polygon faster is an obvious solution, but bearing life imposes an upper limit on polygon speed of rotation. One improvement used in the past is to scan with two beams, which allows the generation of two scans per polygon facet. An example is commonly assigned U.S. Pat. No. 4,686,542, High Speed, High Resolution Raster Output Scanner. The single original beam is separated into two beams which are individually modulated and used to generate two raster output scans per polygon facet. The problem with this arrangement is that a certain minimum distance must be maintained between scan lines at the photoreceptor plane or the beams will interfere with each other to produce artifacts, and this limits the number of lines per inch at the output if the apparatus is being used to generate two consecutive scan lines. In the alternative, a higher density of scan lines per inch can be produced by using two beams to produce non-consecutive scan lines. That is, each two scan lines generated are separated by one, two or more scan lines. Of course, in this case, the display information must be supplied in the proper sequence also. The present invention described below provides a method of generating two non-interacting but closely spaced beams per facet.